The present invention relates to a method of controlling the sizes of dots written on an image-receiving body, an image-writing device employing this method, and an image-forming apparatus including this image-writing device.
Examples of image-forming apparatus to which the invention is applicable include electrophotographic color printers of the tandem type and intermediate transfer type. Like electrophotographic printers in general, these printers have an exposure unit, a photosensitive drum that functions as the image-receiving body, a developing unit, a transfer unit, a fusing unit, and a paper transport unit. The surface of the photosensitive drum is uniformly charged, then illuminated by the exposure unit to form an electrostatic latent image. The latent image is developed by application of toner particles in the developing unit. The resulting toner image is transferred to paper or other printing media in the transfer unit, and is permanently fused with the paper (or other media) by heat and pressure rollers in the fusing unit. A printer of the tandem type has a separate exposure unit, photosensitive drum, developing unit, and transfer unit for each primary color. A printer of the intermediate transfer type has an additional intermediate transfer drum to which the image is transferred one color at a time before being transferred in full color to the paper.
In these types of color printers, it is customary to print each primary color at a particular screen angle. This means that when colors are printed over large areas, dots of each primary color align in a particular direction. For example, dots of a first primary color may align vertically, while dots of a second primary color align diagonally in one direction, and dots of a third primary color align diagonally in another direction.
One type of exposure unit employed in these color printers is a light-emitting-diode printing head, referred to below as an LED head. An LED head includes a linear array of light-emitting diodes (LEDs) disposed in a plurality of LED array chips with a total length equal to or greater than the width of the printing media (e.g., the width of a sheet of paper of standard size). The LED head also includes a plurality of integrated driver circuits or driver ICs that drive the LEDs.
An LED head further includes a lens array such as, for example, the self-focusing or ‘Selfoc’ type of lens array manufactured by Nippon Sheet Glass Co. This is an array of glass cylinders that function as lenses, focusing the light emitted by the LEDs onto the surface of the photosensitive drum.
The LEDs in an LED head may differ in the intensity of the light they emit. It is known art to compensate for such differences by storing compensation parameters in a memory device and controlling the amount of driving energy supplied to the LEDs according to the compensation parameters so that each LED, when driven, delivers substantially the same amount of optical energy to the photosensitive drum.
This known compensation method fails to compensate for dot distortion caused by the lens array, however. Variations in the optical properties of the glass cylinders, non-uniform mounting of the glass cylinders in the lens array, and positional errors in the mounting of the lens array as a whole can cause the dots of light to be focused in a skewed manner, even if their energy remains uniform. Typically, the dots are distorted into ellipses, leading to reduced resolution in the direction of the long axes of the ellipses.
Dots that are distorted in this way can cause visible printing defects, particularly if the distortions are aligned with the screen angle of the dots. A conventional compensation scheme that equalizes the dot illumination energy to within ±2%, for example, is no defense against a lens array that distorts the dots so much as to cause resolution variations of ±10%.